In many optical systems such as photolithography systems, an object such as a reticle is illuminated by a light source. Light from the object passes through a projection or exposure lens which forms an image of the object at an image plane. In photolithography systems, a workpiece such as a semiconductor wafer or flat panel display substrate is mounted on a movable stage at the image plane, and an image of the illuminated reticle is formed by the projection lens on the top surface of the workpiece.
In today's semiconductor and flat panel display industries, increasingly dense circuits are required on wafer and display substrates. The increased circuit density demands smaller feature sizes and line widths in the images projected onto the substrates. Feature sizes in the sub-micrometer range are increasingly in demand.
With such small required feature sizes, even small shifts in the image plane or focal plane of the lens can adversely affect the performance of the system. If the focus shifts such that the workpiece surface moves beyond the depth of focus of the lens, then the reticle pattern cannot be accurately reproduced on the substrate.
Assuming a mechanically stable system, the main cause of focal plane shift is thermal changes induced in the lens by absorption of a portion of the exposure light. The resultant heating induces changes in the refractive indices of the optical elements as well as in their curvatures. The effect is a shift in the focal plane of the lens over time. The time constant of the shift is usually several minutes, which can be of the same order as the time needed to completely expose a substrate. Thus, if the shift is a significant portion of the depth of focus, it can interfere with proper exposure of the substrate.
Other causes of focal plane shift include changes in atmospheric air pressure, temperature and humidity. However, these changes occur slowly and, hence, do not cause significant problems during exposure of a substrate.
Prior systems have attempted to compensate for focal plane shift due to lens heating. In one method, a block of metal is placed in the illuminator where it receives a portion of the light intended for the lens. The block is made to have heating and cooling time constants approximately equal to those of the lens. The temperature of the block is monitored with a thermocouple. From the temperature information, the focal plane shift of the lens is computed and an appropriate compensation is made at the plane of the substrate.
In another method, the focal plane shift of the lens over time at full light intensity is characterized and the data is stored. The actual amount of light that passes through the lens during an exposure is calculated by multiplying the full intensity value by the transmission percentage of the reticle being used during the exposure. From this calculated value, compensation is made at the substrate plane as the exposure is executed.
These prior approaches are essentially modeling approaches in which a simulated focus shift is calculated and for which compensation is made. As with any simulation, inaccuracies are introduced by the imperfection of the model and the absence of actual measurement.
Most photolithography systems also include another type of focus sensor, commonly referred to as a stage sensor. The device is used to determine the position of best focus of the system. One such stage sensor works in conjunction with a grating formed on the reticle. The system illuminator projects light through the grating onto the system projection lens which forms an image of the grating on a detector located on the system stage. The detector can include a single slit aperture over an array of optical detectors or a single detector. Instead of a single slit, a second grating having the same period as the first grating can be used.
This prior art stage sensor determines the position of best focus between the stage and the lens by implementing a series of movements in three dimensions x, y, z, where x and y are orthogonal dimensions in the plane of the stage and z is the dimension along the optical axis of the lens which connects the lens with the stage.
In the best focus position sensing process, at each of a plurality of incremental z positions, the stage is moved in the x and/or y dimensions to scan the aperture or grating on the stage under the image of the reticle grating. The best focus position in the z dimension is the z position which provides the highest amplitude oscillating signal at the detector as the image of the reticle grating is scanned.
While this process is highly effective at setting up the photolithography machine at the proper focus position, it is impractical to use during the exposure procedure to adjust for shift in focus due to thermal effects. It is very time-consuming in that it requires intricate movement in two or three dimensions while the optical signal projected from the reticle is detected and analyzed.